1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount which provides a desired vibration damping or isolating effect, utilizing flow of a fluid contained therein, and more particularly to such a cylindrical elastic mount that has three orifice passages tuned to different frequency ranges, and exhibits different vibration damping or isolating characteristics depending upon a frequency range of input vibrations, by forcing the fluid to flow through a selected one of the orifice passages.
2. Discussion of Related Art
A cylindrical elastic mount is known as a vibration damper interposed between two members of a vibration system, to flexibly connecting the two members or flexibly mounting one of the two members on the other member. The cylindrical elastic mount includes an inner sleeve fixed to one of the two members, an outer sleeve fixed to the other member and disposed coaxially or eccentrically with respect to the inner sleeve with a suitable radial spacing therebetween, and an elastic body interposed between the two sleeves for elastically connecting these sleeves. The elastic mount also has a primary fluid chamber which is partially defined by the elastic body and is adapted to undergo pressure changes upon application of vibrations to the mount, an auxiliary fluid chamber which is partially defined by a flexible diaphragm and is adapted to permit volumetric changes thereof, and an orifice passage connecting these primary and auxiliary chambers to each other. The cylindrical elastic mount as described above utilizes an effect of flow of the fluid, more specifically, the resonance of the fluid flowing through the orifice passage, so as to provide such a high vibration damping or isolating effect that is not achieved only by the elasticity of the elastic body. This type of elastic mount has been favorably used as an engine mount for a motor vehicle, for example.
Generally, the elastic mount, such as an engine mount, receives different kinds of vibrations having different frequencies and amplitudes, and is required to exhibit different vibration damping and isolating characteristics with respect to those vibrations. However, the damping or isolating of the input vibrations based on the resonance of the fluid flowing through the orifice passage is effective only to a relatively narrow frequency range to which the orifice passage is tuned. Thus, the known elastic mount having only one orifice passage is hard to achieve desired vibration damping and isolating characteristics. JP-A-6-74287 (published in 1994) discloses a fluid-filled cylindrical elastic mount having a primary fluid chamber and an auxiliary fluid chamber between which a first orifice passage and a second orifice passage tuned to different frequency ranges are formed in parallel with each other. This elastic mount further includes a rotary valve for selectively permitting or interrupting flow of the fluid through the second orifice passage tuned to the higher frequency range. In this arrangement, the cylindrical elastic mount selectively provides a high damping effect with respect to low-frequency vibrations due to the fluid flow through the first orifice passage, and a high isolating effect with respect to high-frequency vibrations due to the fluid flow through the second orifice passage, depending upon the input vibrations, for example. While this elastic mount is capable of damping or isolating the input vibrations in two different frequency ranges, due to the flow of the fluid through the two orifice passages, the elastic mount used in a motor vehicle, in particular, is strongly desired to exhibit three different vibration damping and isolating effects for input vibrations in three different frequency ranges. That is, the vehicle elastic mount is required to provide a high damping effect with respect to low-frequency vibrations of around 10 Hz, such as engine shake, a high isolating effect with respect to medium-frequency vibrations of about 15-40 Hz, such as those occurring during idling of the engine, and a high isolating effect with respect to high-frequency vibrations of about 60-80 Hz, such as booming noise during a low-speed running of a motor vehicle.
JP-A-6-50379 (published in 1994) discloses a cylindrical elastic mount having a primary fluid chamber, a first auxiliary fluid chamber, a second auxiliary fluid chamber, a first orifice passage tuned to a low frequency range, a second orifice passage tuned to a medium frequency range and a third orifice passage tuned to a high frequency range. The first and second orifice passages are formed between the primary fluid chamber and the first auxiliary fluid chamber, while the third orifice passage is formed between the primary fluid chamber and the second auxiliary fluid chamber. The second auxiliary fluid chamber is defined by a wall having a larger stiffness (or resistance to deformation) than a wall defining the first auxiliary fluid chamber. Upon application of the low-frequency vibrations to which the first orifice passage is tuned, therefore, the fluid is more likely to flow through the first orifice passage than the third orifice passage. Further, valve means is provided for permitting a selected one of the second and third orifice passages to communicate with the corresponding two fluid chambers. Thus, the present elastic mount exhibits different damping and isolating characteristics depending upon the input vibrations, so as to damp or isolate the low-, medium-, and high-frequency vibrations due to flow of the fluid through the first, second and third orifice passages.
In the cylindrical elastic mount as disclosed in JP-A-6-50379 indicated above, however, only one of the second and third orifice passages is selected to communicate with the corresponding fluid chambers. It is therefore considerably difficult for the elastic mount to effectively isolate the medium-frequency vibrations and the high-frequency vibrations at the same time. When the elastic mount receives the high-frequency vibrations with the second orifice passage communicating with the primary chamber and the first auxiliary fluid chamber, in particular, the flow resistance of the second orifice passage becomes excessively high, owing to the high-frequency input vibrations, whereby the isolating effect for the high-frequency vibrations is considerably deteriorated.
In the cylindrical elastic mount as described above, the valve means for selectively permitting the fluid communication of the second and third orifice passages may consist of two switching valves provided in the the second and third orifice passages, respectively, or a single valve having two valve faces which are open to the respective second and third orifice passages. The use of the two valves may result in an increased number of components and complicated structure of the elastic mount, and reduced efficiency in manufacturing the mount. On the other hand, the use of the single valve having the two valve faces may add a limit to a choice of design for the orifice passages, making it difficult to suitably determine the shapes and dimensions of the orifice passages, so as to ensure desired cross sectional areas and lengths of the passages.
Further, in the above cylindrical elastic mount, the wall defining the second auxiliary fluid chamber includes a flexible diaphragm which is formed by closing a window of the outer sleeve with a part of a rubber layer bonded by vulcanization to the inner circumferential surface of the outer sleeve. The flexible diaphragm may otherwise be a separate rubber film-like member provided on the inner circumferential surface of the outer sleeve to expand into the second auxiliary fluid chamber. However, it is considerably difficult to produce the mount having such a flexible diaphragm, and the flexible diaphragm thus formed is likely to be damaged, since it is exposed to the outside of the mount through the window of the outer sleeve.